First, I tried it the hard and expensive way. I bought a Lucas Classic assembly ($92). 1. Tried removing just the switch from the existing bracket. Very painful contortionist experience. Proved impossible to get the new on one (could not get the switch to go under the loosened top bolt. 2. I then removed the assembly from inside the engine compartment. (Two nuts on the firewall) Worked well to remove the whole assembly. Almost impossible to put the new one back on. Ended trying fishing line, and then a wooden dowel to feel for hole location. After a cumulative very painful 8 hours being Houdini, it was in. But the problem remained! A few bad words were ushered. Loudly. Only way to turn the brake lights off was flip the pedal up after braking. (Note this also confuses cruise control and eventually leads to "Engine Failure Protection Mode".

After sleeping on it. I hit on the solution.
1. Bend the switch bracket towards the door (it's robust enough and flexible enough to take it).
2. Cut a 1-3/4" piece of 5/16th PVC tubing.
3. Squirt a dab or two of Gorilla glue inside it.
4. Slide it over the brake activation bar (metal 1/4" bar 90° off the brake pedal stem.
5. Bend the switch bracket back, making sore that the switch lever is engaged by the PVC tube covered bar.

Hey Presto! Problem solved in 5 minutes for about 10¢. Brake lights and cruise control work perfectly. I am both extremely happy and mad as hell at the same time. Tolerance on the switch activation was previously ridiculous. This fix makes on and off defined very clearly.

Hope this helps someone else avoid misery.

(Note the old "Cherry" brand switch still worked, so I now have a spare.)

 

OR

You can double check the trigger part of the switch (the white plastic part). It is actually adjustable. There are 2 separate plastic parts pressed together and joined by small plastic teeth, which allows these parts to be forcibly moved in relation to one another and remain in place after adjustment. IOW, the trigger can be collapsed or expanded to fit the peg on the pedal and properly activate the underlying switch.

The "dirty" part and the whiter part in the image below can slide in relation to each other.

 

The two separate parts are a 'ratchet-style' assy that is designed to set at the correct position when the brake pedal is released to the top of the travel. (at rest)

You will hear a clicking while the parts ratchet into position and then leave it alone. Do NOT PULL the brake pedal UP, just let it return naturally.

Pulling up will over extend the travel and the brake lights will STAY ON.

I replaced many brake light switches in the X100 and X308 cars and did PULL UP once. I had to pry the two parts apart and let the pedal set itself while 'stuffed' under the dash to gain access.(it can be done with a small 'hook-tool' while in-situ)

 

Having the factory microswitches in my wife's 2006 XK8's brake pedal switch assembly replaced with more robust ones fixed this issue for me back in late 2016. Since then, no issues with it....